Illicit clandestine shipment of nuclear explosives, materials that can be employed in the fabrication of nuclear explosives, and materials that can be employed in the fabrication of dirty bombs may constitute a major threat to the peace and security of the world. Such materials may be secreted and smuggled in cargo or other shipments in various containers including ordinary luggage, crates, vehicles, cargo containers, etc. by terrorists, potential terrorists, terrorist supporters, or others. Effective and efficient methods and systems are required for the reliable, non-intrusive, detection of such contraband materials in ports and in other cargo and shipping locations in order to reduce the risk of successful illicit shipments, without unduly impeding the worldwide flow of cargo in a manner that is disruptive of normal commerce. Accordingly, it is especially important that the detection methods not produce large numbers of false positive detection events.
Passive detection methods, as for example gamma spectroscopy of natural decay, have not proven universally effective since many of the materials of interest are not highly radioactive and are relatively easily shielded. X-ray techniques do not readily distinguish between fissionable nuclear materials and innocuous high-Z materials like lead or tungsten that may be legitimately present in cargo.
In addition to passive detection, several approaches to detection have been employed, attempted, or proposed using active techniques employing probing beams.
In one such active technique, an external neutron source has been used to detect fissionable nuclear materials by detecting induced fission events by the neutron multiplication effect of the fission events. However, it has been difficult to discriminate between the probing neutrons and the fission induced prompt neutrons, especially when the energy of the probing neutrons is as high as the energy of the more energetic prompt neutrons from fission or when large containers are involved. Alternative techniques have induced fission events in fissionable nuclear materials with pulsed external neutron sources, then detecting delayed emission of neutrons by fission products, using time delay, as a means of distinguishing the detected signal from the probing neutrons. This delayed neutron signal is a much weaker signal, and is subject to signal-to-noise ratio problems.
In other active techniques, gamma ray probe beams have been employed to induce photofission (γ, f) of nuclear materials with detection of neutrons resulting from the fission events. Scattered gamma rays from the probe beam as well as photo-neutrons (direct (γ, n) events resulting from interaction of the gamma probe beam with fissionable and/or non-fissionable nuclei) induced by the probe beam contribute noise to the detection of prompt neutrons from the fission events, contributing to unreliable or ambiguous detection. Photofission also results in delayed neutron production by the fission fragments, but as with neutron-induced fission, the delayed neutron signal is weaker and detection suffers from noise problems.
It is therefore an object of this disclosure to provide improved systems and methods for detecting fissionable nuclear material in an article with reduced error and ambiguity.
It is a further object of this disclosure to provide improved systems and methods for detecting contraband fissionable nuclear materials by improving discrimination of prompt fission neutrons in the presence of noise-contributing factors.
Another object of this disclosure is to provide systems and methods for analyzing the energy or an energy spectrum of prompt fission neutrons to detect the presence of fissionable nuclear materials in an article.
A still further object of this disclosure is to provide systems and methods for detecting an angular distribution of prompt fission neutrons to detect the presence of fissionable nuclear materials in an article.
Yet another object of this disclosure is to provide systems and methods for using an angular distribution of prompt fission neutrons and an energy distribution of prompt fission neutrons to detect the presence of fissionable nuclear materials in an article.
The objects set forth above as well as further and other objects and advantages of the present disclosure are achieved by the embodiments described below.